Nanoporous Metal–Organic Framework Thin Films Prepared Directly from Gaseous Precursors by Atomic and Molecular Layer Deposition: Implications for Microelectronics

Atomic/molecular layer deposition (ALD/MLD) allows for the direct gas-phase synthesis of crystalline metal–organic framework (MOF) thin films. Here, we show for the first time using krypton and methanol physisorption measurements that ALD/MLD-fabricated copper 1,4-benzenedicarboxylate (Cu-BDC) ultrathin films possess accessible porosity matching that of the corresponding bulk MOF.


Materials and substrates
Cu(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) was synthesized following a previously reported procedure, 1 4benzenedicarboxylic acid (H2BDC, >99.0%) was purchased from Tokyo Chemical Industry Co., Ltd. and used without further purification. Single-side polished p-type (100)-oriented silicon (Si, Okmetic Ltd.) substrates were used for most of the characterization. Samples meant for porosity characterization were deposited on high-aspect ratio (HAR) Silicon pillars as substrate. HAR substrates (staggered square array with pillar diameter of 2 μm, pillar height 50 μm and inter pillar distance of 2 μm) were produced by means of reactive ion etching (Bosch process) using 600 nm silicon oxide patterned by photolithography as the hard mask. For methanol porosimetry, films were deposited straight on SiO2coated 5 MHz AT-cut quartz crystal microbalance sensors (AWSensors).

Thin film deposition
The Cu-BDC films were deposited from Cu(thd)2 and H2BDC precursor powders. The depositions were carried out in a commercial flow-type hot-wall ALD reactor (F-120 by ASM Microchemistry Ltd.), in which the solid precursors were placed in open glass crucibles and heated at 103 °C (Cu(thd)2) or 185 °C (H2BDC) for sublimation. The reactor pressure was ∼3 mbar with nitrogen (99.999%; Parker HPN 5000 N2 generator) used as both the purging and carrier gases.

Thin film characterization
Synchrotron Grazing incidence X-ray diffraction (GIXRD). The crystallinity of the ALD/MLD Cu-BDC films was measured by GIXRD at beamline XRD1 of the Elettra synchrotron (wavelength: 1.4 Å, incidence angle: 0.8°). The diffraction pattern of a ca. 100-nm thick Cu-BDC film onto Si was obtained from 2D detector pixel images using the GIDVis software package. 2 The film thickness was determined through X-ray reflectivity measurements (XRR; X'Pert Pro, PANalytical; CuKα; time per step 6 s).
X-ray reflectivity (XRR). Measurements were performed on a Malvern PANalytical Empyrean diffractometer using a Cu anode (Cu Kα1 = 1.5406 Å; Cu Kα2 = 1.5444 Å) operating at 40 mA and 45 kV with a step size of 0.005 °and a time per step of 6 s. X'Pert Pro software (Malvern PANalytical) was used to fit the data. Densities were deduced from the critical angle θc in the XRR patterns, using equation ρe = (θc 2 π)/(λ 2 re), where ρe is the mean electron density, λ is the Xray wavelength, and re is the classical electron radius. By assuming the elemental composition as pure copperdicarboxylate (C8H4CuO4), the mass density was estimated from ρm = (ρeA)/(NAZ), where A is the average molar mass, NA is the Avogadro constant, and Z is the average atomic number.

Fourier transform infrared spectroscopy (FTIR).
Measurements were performed in transmission mode in the range 400-4000 cm -1 on a Bruker alpha II infrared Spectrometer. The spectrum represents the average of 24 scans at 4 cm -1 resolution after subtracting the blank Si reference spectrum.
Krypton physisorption (KrP). The specific surface area of the sample (in m 2 m −2 ) was calculated from the Kr adsorption isotherms measured at 77 K (Micromeritics 3Flex 3500 gas physisorption instrument). The sample was degassed prior to measurement at 150 °C under vacuum (10 −2 mbar) for 10 h to ensure complete activation. Gas uptake equilibria were measured as described elsewhere. 3 To calculate the specific surface area expressed per unit of substrate area, the total surface area of the MOF (0.51 m 2 ) was divided by the geometrical area of the HAR pillars. The surface area was calculated by applying the BET method in the region 0.001-0.07 P/P0 satisfying the Rouquerol criteria. 4 A Kr adsorptive cross-sectional area of 0.210 nm 2 was assumed for the BET calculations. The BET surface area of the sample was corrected by subtracting the area of a blank measurement (uncoated HAR pillars). The theoretical surface area of the ZUBKEO structure was calculated by Monte Carlo sampling using Zeo++. 5 The Kr hard sphere radius of 1.8 Å 6 at the triple point was used as an approximation for the Kr probe radius at 77 K.
Quartz crystal microbalance (QCM) porosimetry. Methanol adsorption was measured by placing a Cu-BDC-coated sensor into a dedicated QCM cell (X4 AWSensors), followed by exposure to methanol vapor of different concentrations. Before the measurement, the sample was mildly activated in-situ at 40 °C under a nitrogen flow. Methanol vapor was then generated by an in-house built set-up consisting of a mass-flow controller (MFC) feeding nitrogen to a methanolfilled bubbler and another MFC controlling the diluting nitrogen flow. The bubbler was immersed in a thermostatic bath to precisely control the saturated vapor pressure. Both the bath and the QCM cell were kept at 23.0 °C. During the experiment, the sensor resonant frequencies (fn) and bandwidths (Γn) for different overtones (n = 1, 3,5,7,9,11,13) were monitored. As the change in the bandwidth (ΔΓn) was much smaller than the resonant frequency shift (Δfn) for all the measured n (ΔΓn/Δfn < 0.01), the Sauerbrey equation was applied to calculate the change in the layer's mass corresponding to the methanol adsorption. 7 To determine the mass of the deposited Cu-BDC layer, it was first dissolved in Piranha (H2O2:H2SO4 1:3 v/v) and then the fn and Γn of the same bare sensor were compared to those of the sensor after Cu-BDC deposition. Similarly, as ΔΓn/Δfn < 0.01, the Sauerbrey equation was used to calculate the mass of the MOF layer.
Atomic force microscopy (AFM). Samples were scanned under ambient conditions using a Bruker Multimode 8 AFM in tapping mode. Silicon tips with a natural resonance frequency of 300 kHz and with an equivalent constant force of S3 26 N·m -1 (AC160TS-R3, Olympus Corporation) were used. The scan rate was adjusted during the scanning of each image (0.1-1 Hz, 512 samples/line). Data processing and analysis were carried out with Gwyddion software. 8 Table S1. Previously reported Cu-BDC crystalline structures, detailing lattice parameters and data from CCDC database, Cambridge, UK. Two dense (JIBFUV and KAKSUL) and one porous (ZUBKEO) Cu-BDC structures have been identified so far. The JIBFUV structure consists of Cu-octahedral chains bridged with water molecules through hydrogen bonding, while the KAKSUL structure is a three-dimensional non-porous coordination polymer consisting of Cu(OH)2 layers intercalated by terephthalate anions. Finally, the porous ZUBKEO ( Figure S1) structure consists of dinuclear Cu II moieties bridged with BDC linkers forming two-dimensional sheets and one-dimensional pores. A similar phase (PUYREH) was originally reported with DMF occupying the pores and coordinating to the Cu II dimers.  Figure S1. Structure of the ZUBKEO phase (CCDC 1056985) for Cu-BDC viewed along the a axis. Cu, O, C, and H atoms are colored blue, red, grey and white, respectively.